1. Field of the Invention
The present invention relates to semiconductor integrated circuits having a circuit that reduces electromagnetic emissions generated by power supply current without degrading circuit performance. In particular, the invention relates to a choking circuit which has multiple levels of current smoothing.
2. Description of the Related Art
Modern electronic systems often include many integrated circuits, and the rapid switching of many thousands of logic elements causes electromagnetic emissions. Such emissions, when transmitted along power bus conductors, can interfere with the operation of the electronic system. Electromagnetic emissions may also interfere with the operation of nearby electrical equipment due to electric and magnetic fields radiated into space. This is the general problem of electromagnetic interference ("EMI").
In an integrated circuit, emissions are typically generated by the large numbers of transistors that switch in response to a transition of an input clock signal. Such simultaneous switching of the transistors generates high amplitude transient current pulses that must be provided by an external power supply. It is these transient pulses that cause most of the electromagnetic emissions generated by integrated circuits. On the other hand, a highly integrated circuit, such as a system-on-chip, consists of many modularized elements, but not all these elements are enabled simultaneously. Typically, only part of the elements are enabled to perform certain functions at a specific time. Therefore, the current of an integrated circuit drawn from an external power supply depends on the activities of the integrated circuit.
Inventors have proposed a number of solutions to reduce EMI. For example, U.S. Pat. No. 5,561,792 teaches that EMI may be lowered by selecting a lower clock frequency. U.S. Pat. No. 5,422,919 discloses that a pseudorandom code may be added to a digital signal to reduce its EMI. U.S. Pat. No. 5,675,832 teaches that a delay may be added to a signal to lower its EMI.
Another solution (for example, U.S. Pat. No. 5,317,207) has attempted to reduce the generation of electromagnetic emissions by choking the power supply current at every transition of a clock signal so that the amplitude of the transient pulse can be reduced and the pulse energy can be spread over an extended period of time. This reduction is made possible by dynamically modulating the resistive portion of a low pass filter between the external power supply and the power bus of the integrated circuit. Just prior to every transition of the clock signal in the integrated circuit's nucleus logic, the resistive portion of the low pass filter is increased to given value, termed a "choking level", and is then gradually reduced. In response to this resistance modulation, the voltage level on the internal nucleus power bus is necessarily reduced. In this patent, the way of modulating the resistive portion of the lower pass filter is the same at every transition of the clock signal, regardless the activities of the integrated circuit. Hence, it can be referred to as an I.sub.cc current EMI choke with a single choking level.
For the I.sub.cc current EMI choke with a single choking level, the choking level is chosen based on a high activity case of the integrated circuit, that is, when all modularized elements of the integrated circuit are active simultaneously, so that the voltage level on the internal nucleus power bus is not reduced below the level required in the most sensitive circuit operation. On the other hand, this choking level can be too little to reduce EMI efficiently in a case when only part of the modularized elements of an integrated circuit are active, which is the typical case most of the time for a highly integrated circuit.